Titration is generally known as a method of determining volumetrically the concentration of a substance in a subject material by adding a standard solution of known volume and strength (this standard solution is known as the "titrant") to the subject material until a given reaction is completed.
Typically, titration is carried out to the "endpoint", i.e. the point at which the given reaction is complete, to determine the concentration of a given substance in the material. Various potentiometric, colorimetric, and coulometric titration processes are known in the art. Potentiometric determination methods are generally based on the determination of pH, ion activity, redox and other chemical potentials. Uses of titration techniques include determination of the contents of a chemical solution (See, U.S. Pat. No. 4,859,608 to Frueh) or to determine the concentration of certain functional chemical groups in a solution (See, U.S. Pat. No. 3,730,685 to Prohaska).
Titration processes require a high degree of precision, extensive operator interaction and are generally very time corisuining. Further, because the amount of titrant added to a solution to yield a given change will vary dramatically as the electric potential changes, overshooting the endpoint of a titration often occurs. Overshooting the endpoint of a titration results in inaccurate and often useless data or results.
In an effort to increase the accuracy and efficiency of titration processes, automatic titration systems have been developed. Most often these systems take the form of titration-to-endpoint methods mentioned above. In a typical endpoint system, the automatic titrator will produce a recording of the entire titration curve from which the endpoint may be determined.
In the curve-recording type of automatic system, titrant is delivered by means of a motor-driven syringe, or burette, through a capillary whose tip is immersed in a rapidly stirred solution. The motor drive is coupled to the drive mechanism of a recording potentiometer, with the recording chart divisions being directly related to the titrant delivered. The measured potential variations are automatically plotted on the chart, and by inspection of the plotted curve, the entrance and exit of the steep portion of the curve in which an inflection occurs can be located. By taking half the distance between these points, a close estimate of the endpoint is obtained.
One of the key advantages of an automatic titrator is that a large number of similar titrations can be performed in a short period of time, but the requirement for manual determination of the endpoint in each case obviates this advantage. In addition, poor results can occur if there is a slow attainment of equilibrium in the solution during the titration, or if the mixing rate is inadequate. This problem can be overcome to some extent by the use of an "anticipation" technique wherein the rate of titrant addition is automatically slowed as the end point is approached. However, anticipation does not entirely eliminate the problem, for the slower rate itself introduces errors and also reduces the advantage of the automatic system. It should also be noted that in many titrations, the points of inflection (areas of rapid change in electric potential that are indicative of the endpoint) on the plotted curve are very difficult to locate with any accuracy, and this introduces further error in the measurements.
There are many examples of automated titration-to-endpoint processes. U.S. Pat. No. 3,730,685 to Prohaska discloses a computer-controlled process whereby a program is used to analyze the progress of the titration curve by determining the slope of the curve after each incremental addition of titrant. The maximum slope of the curve is calculated to determine the endpoint of the titration.
U.S. Pat. No. 3,769,178 to Rethermal, Jr., discloses an automatic titration process that plots the first derivative of the titration curve in an attempt to more accurately determine the endpoint of the titration.
Other methods of automatic titration to determine the end point of the titration are disclosed in U.S. Pat. No. 4,266,942 to Vandenbossche et al., U.S. Pat. No. 4,302,299 to Ishikawa, and U.S. Pat. No. 4,859,608 to Frueh.
Titration processes may also be used to adjust the electric potential of a subject material. The adjustment may take the form of, for example, pH modification or the modification of the ion concentration in a material. U.S. Pat. No. 4,940,551 to Riggs et al. discloses a process where predetermined dispenses are injected into a fluid stream to create an in-situ titration curve to calculate one total dispense to obtain a desired pH. However, this system is slowed by the fact that the titration curve must be created before adjustment may commence.
The known processes utilizing automatic titration have been unable to completely solve such problems as excessive overshooting of the aim or endpoint, slow titration times or the requirement of operator interaction. Further, the known processes utilizing automatic titration are often restricted to pH applications or are designed for application only in titration-to-endpoint systems.